In order to obtain low resistance fine wirings, wirings are formed by a damascene method by using copper (Cu) or copper alloy as wiring material. According to a general damascene method, a barrier metal film is formed on an interlayer insulating film, covering a wiring trench formed in the interlayer insulating film, a seed film is formed on the barrier metal film, and a Cu film for example is formed by a plating method on the seed film. Unnecessary portions of the Cu film and barrier metal film on the interlayer insulating film are removed by chemical mechanical polishing (CMP) to leave wirings in the wiring trenches.
If a conductive layer such as a barrier metal film is left on the interlayer insulating film after polishing, this conductive layer may cause leak between wirings. In order to prevent the conductive layer from being left on the interlayer insulating film, an upper portion of the interlayer insulating film is cut to some depth during CMP for the conductive layer to perform so-called over polishing.
An interlayer insulating film using low dielectric material having a dielectric constant of, e.g., 3.0 or lower has been proposed recently in order to reduce a parasitic capacitance (e.g., refer to Japanese Patent Laid-open Publication No. 2006-156519). The low dielectric constant film of this type contains methyl groups (CHx) and the like and is hydrophobic. Therefore, the surface of the low dielectric constant film have a tendency to repel CMP polishing slurry, and polishing is hard to progress.
An interlayer insulating film having the structure that a hydrophilic cap film made of, e.g., silicon oxide (SiO2) is formed on a hydrophobic low dielectric constant film has therefore been proposed. The cap film is over-polished. The low dielectric constant film has low tight adhesion to the underlying insulating film, and is likely to have film stripping. Polishing is stopped by leaving the cap film to some thickness from the viewpoint of preventing film stripping.
However, since silicon oxide (SiO2) has a dielectric constant of over 3.0, if this film is left on the low dielectric constant film, parasitic capacitance between wirings becomes high. In order to avoid high parasitic capacitance, techniques of directly polishing a hydrophobic low dielectric constant film without using a cap film have been developed.
In polishing an interlayer insulating film using a hydrophobic low dielectric constant film, polishing slurry has high wettability in an area where wirings are disposed densely, whereas polishing slurry has low wettability in an area where wirings are disposed coarsely. In the area where wirings are disposed coarsely, dishing and erosion are likely to occur. Dishing and erosion do not occur only when an interlayer insulating film using a hydrophobic low dielectric constant film is polished, but dishing and erosion may occur when a polishing target having a plurality of different materials such as an insulating film having a buried conductive Cu film is polished.
Techniques of providing a uniform wiring density in a wafer plane by disposing dummy wirings between wirings have been proposed to suppress dishing and erosion. Parasitic capacitance is, however, formed between dummy wirings and wirings, and a wiring delay occurs.